The present invention generally relates to an image signal processing method and apparatus, and particularly to an image signal processing method and apparatus in which an image signal supplied from an optical sensor can be corrected.
In a general image scanner, a light source illuminates a wide region including a reading point on a document, a light reflected at the reading point being directly incident to an optical sensor of the image scanner. However, when the light source illuminates the wide region, a light which is reflected at a point other than the reading point on the document can be incident to the optical sensor via some parts of the image scanner. That is, each point other than the reading point on the document can be a second light source, so that an image signal output from the optical sensor is affected by a light from the second light source. A phenomenon in which the light deflected at each point other than the reading point on the document affects the image signal output from the optical sensor is referred to as a secondary light source effect. When the secondary light source effect is generated, an output characteristic of the image scanner deteriorates, and particularly a characteristic for representing gradation of an image remarkably deteriorates.
A description will now be given of a principle of the secondary light source effect in a conventional image reading apparatus with reference to FIGS. 1A and 1B. An image reading apparatus shown in FIGS. 1A and 1B scans a document in a line.
Referring to FIG. 1A, a document 2 is set on a supporting plate 1 which is transparent. The document 2 is illuminated via the supporting plate 1 by a light source 3. A light reflected at each reading point on a reading line 4 is incident to an image information detector 5. The reading line 4 extends in a direction perpendicular to the surface of a paper on which the drawing (FIG. 1A) is shown. The document 2 is scanned by the light source 3 and the image information detector 5 in a direction perpendicular to the reading line 4, so that the reading line 4 continuously moves and the image information corresponding to the whole document 2 is obtained by the image information detector 5.
In this image reading apparatus, an image can be read only on the reading line 4. However, under a condition in which the light source 3 illuminates a point 6 other than the reading point on the reading line 4, when the point 6 on the document 2 is white, a light which is reflected by the point 6 can travel via a construction member 7 to the image information detector 5, as shown by a dashed line in FIG. 1A. In addition, the light reflected at the point 6 on the document 2 is incident to the light source 3, and then the light reflected by a reflector of the light source 3 can also be projected onto the reading line 4 again. When the light reflected by the point 6 other than the reading point on the document 2 is incident to the image information detector 5, the secondary light source effect is generated. On the other hand, in a case where the point 6 other than the reading point on the reading line 4 is black, as shown in FIG. 1B, even if a light is incident to the point 6, the light is not reflected by the point 6. In this case, the secondary light source effect is not generated. That is, even if the reading point on the document has a constant density, the image signal corresponding to the reading point obtained by the image information detector 5 varies in accordance with the density distribution in a region around the reading point.
FIG. 2A shows an image signal output from the image information detector 5 in a case where the image reading apparatus reads an image for one line in a wide white region on the document. FIG. 2B shows an image signal output from the image information detector 5 in a case where the image reading apparatus reads an image for one line across a 10 mm wide white band region surrounded by a black region. In FIGS. 2A and 2B, the axis of ordinates denotes a level (intensity) of the image signal and the axis of abscissas denotes a position of a pixel. When the image reading apparatus reads an image at a white point in the white region, the secondary light source effect is generated as has been described above. That is, the image signal shown in FIG. 2A includes a component corresponding to the secondary light source effect. On the other hand, when the image reading apparatus reads an image for one line across the white band region surrounded by the black region, the secondary light source effect is hardly generated on the reading line. That is, the image signal corresponding to each point in the white band region, shown in FIG. 2B, does not include a component corresponding to the secondary light source effect. Thus, a level (B) of the image signal corresponding to a white reading point shown in FIG. 2B is several percent less than a level (A) of the image signal corresponding to a white reading point shown in FIG. 2A. The level of the image signal corresponding to the white reading point decreases, for example, from the level (A) shown in FIG. 2A to the level (B) shown in FIG. 2B by 7.1% [=(A-B)/A x100].
Conventionally, to prevent the above secondary light source effect from being generated the following methods disclosed in Japanese Patent Laid-Open Publication Nos. 1-267531 and 1-221976 have been proposed.
In the method disclosed in Japanese Patent Laid-Open Publication No. 1-267531, a shading mechanism is provided adjacent to the supporting plate which supports the document so that a point other than the reading point on the document is not illuminated by the light source. In the method disclosed in Japanese Patent Laid-Open Publication No. 1-221976, a shading plate having a slit is provided between the light source and the supporting plate on which the document is set. Then a light emitted from the light source is projected via the slit on the shading plate onto a restricted region on the document.
However, in the above method disclosed in Japanese Patent Laid-Open Publication No. 1-267531, as the shading mechanism must be provided adjacent to the supporting plate, the cost for manufacturing the image reading apparatus increases. Further, as a part of the light directly illuminating the document is also shaded by the shading mechanism, the amount of light projected onto the reading point on the document is decreased, so that a signal-to-noise ratio of the image signal output from the image reading apparatus deteriorates and a reading speed has to be decreased. In addition, in the method disclosed in Japanese Patent Laid-Open Publication No. 1-221976, it is impossible to prevent the secondary light source effect from being generated on the reading line.